The present invention relates to security alarm systems and, in particular, to an improved, relatively high gain, band pass filter, which requires relatively low current and power and which finds application in a glass breakage sensor of a security alarm system.
A peculiar property of most window glasses, especially various materials used in pane glass, is that the glass upon striking a hard surface, such as when broken, demonstrates a relatively strong frequency component in a range centered at approximately 6 KHz. This characteristic facilitates the construction of security sensors or transducers which are capable of monitoring a related narrow band of frequencies for signals of sufficient duration. Alone or in combination with data from other system sensors, an alarm condition can thereby be ascertained for a secured premises.
In their most basic construction, and for a hard wired security system, a variety of conventional circuit components may be used to effect an appropriate sensor design. Typically, such sensors would include a transducer, band pass filter, detector and amplification means for driving a related annunicator. Such a sensor is typically also not designed to be power sensitive, due to the availability of relatively unlimited power from the system power supply.
For wireless, battery powered sensors, however, greater concern surrounds the passive power requirements of maintaining such a sensor in an operable condition for long periods of time. This is of special concern for glass breakage sensors requiring active, high gain filter circuits, such as that of the present invention.